A manganese oxide (composition formula: MnO2) can exist in an alpha, beta, epsilon, gamma, delta, R and lambda-type crystal structures, and has a property which differs physically or chemically depending on the relevant crystal structures. Among these, the R-type is referred to as a ramsdellite-type crystal structure, and has a structure referred crystallographically to as an ortholombic type. Since a manganese dioxide having this ramsdellite-type crystal structure adsorbs a metal complex ion or palladium complex ion in a water, it is an effective functional material for resource recovery and catalyst synthesis (see, for example, Patent Literatures 1, 2 and 3).